Although the present invention is not limited to the field of ion implanters, this field corresponds to a contemplated application and provides a useful context for understanding the invention. Hence there follows a description of ion implanters.
Ion implanters are well known and generally conform to a common design as follows. An ion source produces a mixed beam of ions from a precursor gas or the like. Only ions of a particular species are usually required for implantation in a substrate, for example a particular dopant for implantation in a semiconductor wafer. The required ions are selected from the mixed ion beam using a mass-analysing magnet in association with a mass-resolving slit. Hence, an ion beam containing almost exclusively the required ion species emerges from the mass-resolving slit to be transported to a process chamber where the ion beam is incident on a substrate held in place in the ion beam path by a substrate holder.
Often, the cross-sectional profile of the ion beam is smaller than the substrate to be implanted. For example, the ion beam may be a ribbon beam smaller than the substrate in one axial direction or a spot beam smaller than the substrate in both axial directions. In order to ensure ion implantation across the whole of the substrate, the ion beam and substrate are moved relative to one another such that the ion beam scans the entire substrate surface. This may be achieved by (a) deflecting the ion beam to scan across the substrate that is held in a fixed position, (b) mechanically moving the substrate whilst keeping the ion beam path fixed or (c) a combination of deflecting the ion beam and moving the substrate. For a spot beam, relative motion is generally effected such that the ion beam traces a raster pattern on the substrate.
Our U.S. Pat. No. 6,956,223 describes an ion implanter of the general design described above. A single wafer is held in a moveable substrate holder. While some steering of the ion beam is possible, the implanter is operated such that ion beam follows a fixed path during implantation. Instead, the wafer holder is moved along two orthogonal axes to cause the ion beam to scan over the wafer following a raster pattern.
The above design is particularly suitable for serial processing of wafers where a robot must unload a processed wafer before loading a new wafer to be implanted. Loading and unloading wafers between each implant causes an undesirable delay.
Our U.S. Pat. No. 6,555,825 describes an ion implanter having a twin scanning arm arrangement shown in FIG. 1. Each scanning arm has a motorised hub unit A that can rotate about an axis X1 between a scanning position shown on the right and a loading position shown on the left. A hollow arm B is rotatably mounted to the hub unit at one of its ends so as to be able to turn about axis X2 to effect scanning of a wafer through an ion beam. The other end of the arm is provided with a wafer holder C. Wafer holder C can rotate about axis X3 to allow the orientation of the wafer to be varied.
The construction and arrangement of each scanning arm is such that when the arm is in the loading position, it is above the axis X1 and therefore above the path of the ion beam D. This has particular significance because the wafer holder can be loaded and unloaded without any undesired effects due to the presence of the ion beam. At the same time, the hub unit can be rotated through 90° to convey the wafer holder to the scanning position where a wafer may be scanned through the path of the ion beam. As shown in dotted outline, the wafer may be moved on the arm from position P1 down through lower positions to position P2. In this ion implanter, a ribbon ion beam is used such that this movement sees the entire wafer implanted.
Provision of two such scanning arms allows one scanning arm to be used to scan a wafer while the other scanning arm may be positioned for concurrent loading and unloading of wafers. Hence, as soon as an implant is complete for one wafer, another wafer is ready for implant on the other scanning arm. Such a scanning arm arrangement is not capable of producing linear raster scans.
Another disadvantage of such radial scanning arrangements is the resultant non-uniform dose characteristics arising from differences in scan speed across the width of the wafer. This is because the closest edge of the wafer to the pivot scans more slowly than the outer edge, causing a higher dose on that side of the wafer.